Top Drives have been commonly used on drilling rigs, since the 1980's offshore and since the 1990's on land. Despite numerous refinements over the years, these machines have a sub-optimal reliability record, as affected by the following considerations:                1. The top drive represents a single-point failure on a drilling rig. There is no inherent back-up capability.        2. The drilling rig environment affords poor access for preventative maintenance. Most of the time, the top drive is either moving or at high elevation or both.        3. The machines have sufficient complexity to disqualify or deter wellsite personnel from many service functions.        4. Sub-systems on the top drives typically lack redundancy.        5. Some typical sub-systems, especially hydraulic rotary unions, remain prone to failure, despite years of refinements.        6. Some sub-systems, especially service loops, are inevitably susceptible to external damage in the drilling rig environment.        7. Repair of some major sub-systems, especially gear reduction drives, requires substantial machine disassembly and specialized technicians. A gear reduction repair typically requires at least 24 hours plus mobilization time for a technician and parts (many days in remote locations).        8. Gear reduction drives are also dependent on the proper operation of lubrication circulation systems which typically have significant complexity, numerous failure modes, periodic maintenance requirements and no redundancy.        
To improve top drive reliability, it is desirable to improve the simplicity, redundancy and serviceability of the top drive sub-systems, including:                1. Better facilitate preventative maintenance by wellsite personnel.        2. Improve diagnostic tools for wellsite personnel.        3. Include full or partial redundancy of sub-systems.        4. Modular sub-systems for ease of replacement.        5. Sub-systems externally serviceable/repairable, not buried in the machine.        6. Sub-systems understandable & serviceable by rig personnel.        7. Eliminate critical sub-systems such as lubrication circulation.        
Overhead oil leakage from top drives is often a significant problem on drilling rigs because of occupational, environmental & safety reasons and the necessary time and cost of diagnosis and repair of oil leaks. Elimination of oil lubrication systems required for gear reduction drives would entirely avoid a primary source of oil leakage.